Comparison of effects of aging on fracture of 9Cr–1Mo and 2·25Cr–1Mo steel Part 1 Quenched and tempered material

Abstract

A 9Cr–1Mo (wt- %) steel and a 2·25Cr–1Mo steel have been investigated in the quenched and tempered condition and after subsequent aging at 550°C for either 1000 or 5000 h. Transmission electron microscopy (TEM) was used to characterise the microstructures produced by each heat treatment. The ductility and fracture resistance were measured using ambient temperature tensile tests and Charpy tests at various temperatures, respectively. TEM was also used to monitor void nucleation during tensile deformation. Differences in the ductility of the steels before aging and their different responses to aging, which decreases the ductility of the 9Cr–1Mo steel while leaving that of the 2·25Cr–1Mo steel unaffected, are attributed to the different void nucleation characteristics of the precipitates in each steel. Differences in ductile–brittle transition temperature (DBTT) before aging are discussed in terms of differences in the lower shelf fracture mode of each steel and the possibility of crack nucleation at carbide clusters. Segregation of P during aging is considered to be responsible for the aging induced increase in DBTT in each steel, but it is suggested that interaction between P and the carbides in the 9Cr–1Mo steel might modify the segregation characteristics in that material.

MST/1232a